This invention relates to the purification of gas streams and more particularly, of those gas streams which result from the production of semi-conductor devices and which contain noxious substances, especially compounds of arsenic or phosphorus.
In our patent specification WO 89/11905 we describe processes and apparatus for the scrubbing of noxious substances from exhaust gases, particularly those emanating from semi-conductor production chambers in which the noxious substances may themselves be used either as the etching gases used in the production or in the plasma enhanced chemical vapor deposition (PECVD) on to semi-conductor materials.
The process described in our prior patent specification comprise contacting the exhaust gases with:
i) a silicon stage preferably having a purity of more than 90 % and most preferably more than 97%, for example a crystalline or polycrystalline material with a purity of about 98.5%; the silicon may be mixed with "inert" substances such as silica or alumina or with lime (CaO) or magnesia (MgO) or even be alloyed with one or more of the metals chromium, manganese iron, cobalt or nickel, for example in the form of ferrosilicon or manganese-silicon or possibly silicon carbide. PA1 ii) a calcium oxide stage in which the calcium oxide may be in the form of calcium oxide itself, lime, soda lime (calcium hydroxide with sodium hydroxide or potassium hydroxide), any form of which may also contain "inert" materials such as graphite (or coke), limestone (CaCO.sub.3), magnesia MgCO.sub.3 or slaked lime (Ca(OH).sub.2). Overall this stage can be represented as a calcium oxide or a calcium oxide compound or a calcium oxide containing material or a soda lime containing medium. PA1 iii) an optional stage in which the active ingredient is a copper oxide (CuO or Cu.sub.2 O) which may be present on calcium silicate supporting medium or on a calcium oxide/calcium silicate supporting medium. PA1 i) the silicon stage is heated above 200.degree. C. and preferably maintained between 350.degree. and 500.degree. C. PA1 ii) the calcium oxide stage is heated above 100.degree. C. and preferably maintained between 250.degree. and 550.degree. C. PA1 iii) the copper oxide stage is maintained between 150.degree. C. and 600.degree. C. preferably between 200.degree. and 400.degree. C.
An addition of copper (or a copper rich material) to the silicon stage may be beneficial.
The processes are operated at elevated temperatures. In particular:
These different stages may conveniently be placed in apparatus comprising a single cylindrical tube, and with the stages spaces apart within the tube, about which are positioned a plurality of heating means, thermal insulation means and temperature sensing means, for example thermocouple devices, along the length of the tube.
The processes are basically designed to be general purpose ones for the elimination of substantially all the noxious gases emanating from semi-conductor production chambers in particular by rendering the gases harmless by chemical reaction in one or more of the stages.
In certain isolated circumstances, however, there may be a need to ensure that particularly noxious substances, or process reaction products from those substances, are fully eliminated from the gas stream.
In the case of the hydrides of arsenic and of phosphorus, i.e. arsine and phosphine respectively, these tend thermally to decompose at the preferred operating temperatures of the silicon and calcium oxide stages to form elemental arsenic and phosphorus respectively and these also tend to remain in the vapor phase at such temperatures.
Our prior specification recommends the use of the optional copper oxide stage as the hydrides themselves as well as the elemental arsenic and phosphorus will react with the copper oxide stage to form copper arsenate and copper phosphate (both solids) respectively.
The present invention is concerned with the provision of a further means to ensure that these very noxious substances are eliminated and/or rendered harmless.